Furnace cycles to often

DRAZOR · 12-25-2007, 02:21 PM

I have a Comfortmaker RPJ gas unit (around 1984 model) . The unit is cycling every 10-13 mins and it is not that cold out here in Ohio yet ( 30 deg ) . Here is what happens , the stat calls for heat and the draft motor starts to run , the burners light and stay lit for 3-4 mins , just before the burners and draft motor kick off the blower starts to move air through the ducts and shuts off in about 1 minute . The thermostat is a digital unit and I don't think the furnace is putting out enough heat to satisfy the stat causing the frequent cycling . Could it be the limit switch needing adjustment ? . Thank you for any responses .

bigMikeB · 12-25-2007, 03:06 PM

There may be a heater i n the limit that isn't working any longer. You may just want to replace the fan-limit switch if it is original.

DRAZOR · 12-25-2007, 03:27 PM

I pulled the cover off and it looks like the settings on the switch are 85-115-160 . Do these sound about right ? . It is more than likely the original switch so I might just go ahead and replace it .

USP45 · 12-26-2007, 08:36 AM

Sounds to me as if this is the limit switch not working.
First off let me explain what it does so you know why its not working. The lower setting tells the blower fan to shut off so as not to blow cool air into the house. The second or middle temp tells the fan when the combustion chamber is hot enough to start blowing air. The third and highest setting is to shut the entire furnace down. 160 seems a bit low, I usually have it set at 200, BUT it also depends on the manufacturer. DO NOT EVER set the third setting any higher than the original setting because if for some reason the furnace should get stuck and keep running, or the blower does not come on and blow the heat off the combustion chamber then it could burn a hole in it causing a fire. replacing it is very easy. Make sure that the new one is of the same lenth as there are different lenth switches.

bigMikeB · 12-26-2007, 12:39 PM

140 on, 120 off and 190-200 limit are more normal for that unit.

DRAZOR · 12-26-2007, 03:15 PM

Any ideas on who might carry this switch ? .

bigMikeB · 12-26-2007, 03:55 PM

I don't know about your area but suppliers in my area won't sell to home owners.

DRAZOR · 01-03-2008, 03:14 PM

I replaced the switch and all is back to normal . Thanks to the above posters for your input and to a great site for getting the info I needed ! .